An adhesive agent containing a thermoplastic block copolymer as a main component has been used in disposable products typified by a paper diaper and a napkin and, particularly, a hot-melt adhesive agent based on a styrene class block copolymer has widely been used. For example, a paper diaper is produced by bonding a polyethylene film with other members (for example, a nonwoven fabric, an elastic material such as a natural rubber, a water-absorbing paper, etc.) using a hot-melt adhesive agent. The hot-melt adhesive agent can be applied to various members using various methods and, even when using any method, the hot-melt adhesive agent is melted by heating so as to obtain an appropriate viscosity, and then the molten adhesive agent is applied to various constituent members in a dot, linear, stripe, spiral or sheet form.
It is now required for the paper diaper to improve drapeness thereof, and a study has been made in improving flexibility and drapeness of the paper diaper by more thinning a polyethylene film or the above-mentioned various members such as a nonwoven fabric. Thinning of various members significantly reduces material costs. However, thinning of the polyethylene film may cause a problem that heat resistance deteriorates and application of a high-temperature (not lower than 150° C.) hot-melt adhesive agent leads to melting of the polyethylene film or formation of wrinkles of the polyethylene film. Therefore, adhesive agent manufacturers have made a progress on the development of a low-temperature-applicable hot-melt adhesive agent which is capable of applying at low temperature (not higher than 140° C.).
Taking workability and environmental aspect in the case of application of the hot-melt adhesive agent into account, manufacturers producing a paper diaper and a sanitary good strongly desire lowering of the viscosity of the hot-melt adhesive agent. The hot-melt adhesive agent commonly comprises a base polymer and a plasticizer, and a study has been made in lowering the viscosity of the hot-melt adhesive agent by a method in which the amount of the base polymer is decreased to thereby increase the amount of the plasticizer. However, the production of a paper diaper using a low viscosity hot-melt adhesive agent produced using such method may cause a problem that the balance between an adhesiveness to a polyethylene film which composes members of the paper diaper and a retention force (cohesive force) is deteriorated, and the softening point is excessively lowered.
JP 2004-137297 A discloses a hot-melt adhesive agent including a linear type styrene block copolymer, a tackifier resin and a plasticizer. The hot-melt adhesive agent of the literature has a low viscosity, and suitable for applying at low temperature, however, is not sufficient in adhesiveness at low temperature.
JP H5(1993)-311138 A, JP 2006-8947 A, and JP 2010-536957 W mention hot-melt adhesive agents including a radial type styrene block copolymer. However, the hot-melt adhesive agents of the Patent Literatures JP H5(1993)-311138 A, JP 2006-8947 A, and JP 2010-536957 W have high melt viscosities, and are not suitable for conducting application at low temperature. These are, however, insufficient in any of peel strength at low temperature, retention force or tack, which are the essential adhesive performances required for the hot-melt adhesive agents for use in a disposable product.